Sound absorber



April 1959 PER VILHELM BRUEL EI'AL 2,882,989

SOUND ABSORBER I 2 Sheets-Sheet 1 Filed Nov. 16, 1956 INVENTORS P, K Drue L C141: Tejlzz B '1 O y I I I ATTORNEY-3 April 21, 1959 PER VILHELMBRUEL ETAL SOUND ABSORBER 2 Sheets-Sheet 2 Filed Nov. 16, 1956 INVENTORSP 1 .131 L Le C 4.15. Tejuar ATTORNEY3 United States Patent 2,882,989SOUND ABSORBER Per Vilhelm Bruel, N aerum, Denmark, and Carl-Axel ElofTegner, Stockholm, Sweden Application November 16, 1956, Serial No.622,581 Claims priority, application Sweden November 16, 1955 6 Claims.(Cl. 181-33) It is known to use sound absorbers based upon the principleof resonance in order to reduce the reverberation time in places with ahigh noise level, for instance, offices and industrial premises, and inplaces where it is desirable, due to special circumstances, to have ashort reverberation time, for instance, in conference rooms,auditoriums, and so on. The most common procedure is to apply so calledacoustic panels in the ceiling of the room. Such panels arecomparatively expensive and besides their mounting call forcomprehensive, timeconsuming, and expensive preparatory work, and alsogreat painting costs. Furthermore, the method is limited by the factthat the acoustic panels, being flat and rigid, may only be applied on aplane base structure.

An acoustic absorber designed for highest efficiency comprises aperforated front panel mounted some distance away from the solid backwall and with a layer of porous absorbing material applied to theinterior side of the front panel. The main principle of its function isthat the vibrations of the volume of air caught between the front paneland the back wall are obstructed by the acoustic impedance which theabsorbing material olfers to the movements of the air particles, part ofthe sound energy being converted into heat energy. The ratio between thesize of perforations and in some degree also the thickness of theperforated panel on one hand, and the volume of air enclosed behind thesame unit area on the other hand will decide within which frequencyrange the greatest absorbing effect will appear. If the air volumecaught behind a certain unit area is great, maximum absorption will beobtained at the low end of the audio frequency spectrum. If said volumeis small, maximum absorption will be obtained at the right end of thespectrum.

A generally applicable absorber must not have a pronounced maximum ofabsorption but its absorption curve should extend as even as possible.This requires a varying ratio between the area of the front panel andthe air volume present at different spots behind the surface oftheabsorber. Moreover, the absorbing effect should be satisfactory overas great a range of the frequency spectrum as possible. In this respectthe greatest difficulty is to obtain a sufficient absorption at lowfrequencies since these require a comparatively large volume of enclosedair.

The most favourable absorber of this kind from a theoretical point ofview is a hollow body with a large cubic capacity in relation to thecovered surface and having walls which, firstly, are rigid-in order notto irresistently follow the vibrations of the air and, secondly, aredesigned in such a manner in respect to the absorption that theiracoustic impedance will attain approximately 41 acoustical ohms persquare centimeter. An acoustical ohm is the unit of acoustic impedanceand is expressed, in the c.g.s. system as g. cm.- sek" (Per V.Briiel,.Lydisolation och Rumakustik (Sound Insulation and Acoustics ofRooms), Trans. No. 55, 1946, M.l15. of Chalmers University ofTechnology).

By itself there is nothing new involved in designing an absorber in theshape of a hollow body. It has been proposed in prior art to designabsorbers as hollow bodies with walls comprising perforated material,such 2,882,989 Patented Apr. 21, 1959 as a shell with a rear porousabsorbing material of various kinds. Hence, the external wall of theseknown absorbers shaped as hollow bodies only functions as a carrying andcovering member for the porous absorbing material proper.

The main object of the present invention is to provide a sound absorberin which the acoustic impedance necessary for the sound absorption isestablished already by the front panel itself which is made of thinmaterial such as cardboard, metal foil, or the like, having very smalland very narrowly spaced holes, apertures or perforations. Such a soundabsorber constructed in accordance with the absorption principle is theobject of a co-pending application filed by the same applicants. Inapplying this principle of absorption, there is no need for any special,porous absorbing material, which will involve a considerable simplifyingand saving of costs. As a matter of course there is on the other handnothing to prevent applying a porous absorbing material in an absorberdesigned according to the present principle if a special effect isdesired for some particular reason.

In order to render the thin material rigid enough not to follow thevibrations of the sound, the absorber according to the invention isconstructed as a cone, part of a sphere, part of an ellipsoid or anotherhollow body of revolution with a straight or curved generatrix, saidbody being pressed against a solid surface, for instance a ceiling or aWall, by means of a resilient fastening member; the absorber may alsopossibly comprise a combination of several such bodies. According to afurther feature of the invention one end of said resilient attachment isconnected to a support or attachment in the ceiling or the wall whilethe other end is connected to a wire or the like, forming a hook or thelike which is adapted to engage the rim portion of an opening of saidhollow body of revolution.

Absorbers designed as hollow bodies of revolution are known bythemselves but in none of these cases the capacity of the absorber as ahollow body of revolution (or perhaps as having a curved shape) isdependent on the necessity of the wall of the absorber to obtainadequate rigidity, as this wall already by itself is sufficiently thickas to render the required rigidity, irrespective of the shape.

According to the present invention the simplest shape of an absorber isthe conical one, said shape being accom plished by joining together oneor more flat sheets of the perforated basic material, said sheetssubstantially having the form of sectors of a circle with a totalperipheral angle of less than 360, and the joining together beingeffected along the radial edges of the sheets. By providing the cornersof the sector elements facing the tip of the contemplated cone with anotch, an opening is formed at the assembling for introducing of aresilient fastening member which holds the absorber on to the ceilingafter said member has been caused to engage the edge portion about thisopening. The absorber may be assembled of one or more sector elementshaving the same or differing arc lengths, and the number of sectorelements of the absorber may be varied according to the desired topangle of the cone. As an important advantage of the conical shape, itmay be pointed out that the basic material, comprising the flat sectorelements, is easy to pack up and transport and that assembling andmounting on plane as well as on curved surfaces readily may be carriedout by unskilled personnel without any preparatory work.

As the absorber is constructed of a fiat and thin basic material whichwill obtain its curved shape when assembled into a cone or another bodyof revolution and will obtain its rigidity when pressed against thecarrying surface, the fastening device itself constitutes an integratingpart of the invention.

In order to further explain the invention some embodiments thereof willnow be described with reference to the accompanying drawings. Inconnection therewith further advantages associated with the inventionwill be evident.

Figure l is a central sectional elevation of a sound absorber designedaccording to the invention;

Figure 2 is a top plan view of the same absorber in a flattened stateconsisting of sector shaped, substantially fiat sheets or disks, thetotal peripheral angle of which being less than 360;

Figure 3 is a fragmentary cross sectional elevation along the lineIII-III of Figure 2;

Figure 4 is a diagrammatic cross sectional elevation of anotherembodiment of the absorber;

Figure 5 is a perspective view of part of a further embodiment; and

' Figure 6 is a diagrammatic cross sectional elevation of the lastmentioned embodiment in an installed position.

The sound absorber 1 may, for instance, comprise three annular sectorshaped elements 3, 4 and 5, each provided with perforations, aperturesor slits 2, which for instance may be grouped into long and narrowtriangular or otherwise shaped fields so as to cause at the same time adecorative effect. However, the invention is not confined to thisembodiment. The three sector elements 3, 4, 5, cover each a centre angleless than 120 and their radial edges 3a and 4a are folded. Abuttingedges 3a, 4a of adjacent sector elements are joined together in pairs bymeans of U-shaped channels 6 or spring clips slipped onto the edges andserving as clenching or fasting means. A conical screen will be formedwhen the edges of sector elements 3 and 5, shown on the left of Figure2, are brought together and joined or assembled in the manner described.The apex portions or corners 7 at the top of the sector elements arepunched away so that the conical screen formed after joining togetherthe sector elements will obtain a corresponding opening at the top. As amatter of course the invention is by no means confined to the number ofsector elements shown in the drawing or to the shape of these elements.

In the case illustrated in Figure l the absorber is attached underneatha ceiling 10 by means of a fastening device comprising a helical spring14, the top end of which merges into a loop engaging a wire 12 whichforms a loop at its central part. The free parts of the wire also formloops 13 through which fastening pins 11 pass, driven obliquely into theceiling 10. One end of helical spring 14 also forms a loop through whicha wire 15 is threaded, the fastening end of which at the helical springis bent backwards and twined around wire part 15 and at the outer partbent into a hitch or fastening hook 16. In installing the screen, wirepart 15 is caused to protrude through opening 7 in the apex or top ofthe screen, and a small shield or hood 8 in the shape of a truncatedconical envelope surface in put on to this top; wire part 15 beingpulled out through opening 9 of said envelope surface. On pulling wirepart 15, fastening hook 16 is caused to hook on to the edge portionabout opening 9 of hood 8 while simultaneously stretching spring 14,whereupon the freely protruding part of wire 15 may be cut off. Thissimple fastening device will retain and press the screen effectivelyagainst the ceiling and the same fastening device may be usedfor variousscreen sizes.

By the described assembling and fastening method, the sector elementsmay be readily mounted and dismounted again without being damaged. Thisinvolves the eminent advantage that absorbers with different acousticproperties may be obtained from one single type of basic material andthat the design which in the best way solves the actual problems ofabsorption may be worked out by practical tests without damagingabsorbers or ceilings. For instance, by only utilizing sector elements 3and 4 a more acute cone angle and a larger volume of air in relation tothe surface are obtained. The maximum value of absorption is displacedtowards lower frequencies in this embodiment.

Furthermore, according to Figure 4 a cylindrical ring 17 may be arrangedfor screen 1, said ring having approximately the same width as the baseof the screen and possibly being provided with one or more depressions18, 18a, extending around the ring. This ring 17 is applied betweenscreen 1 and the ceiling so that the screen will rest in or adjacent tothe depression 18, if any. In conjunction with this ring two screens,one Within the other, may be utilized; one (indicated by lines 1a) atthe base 18a of the ring and the other 1 at the top edge of the ring orin the aforesaid depression 18. All these alternatives will involve achange of the ratio between surface and volume and thus a displacementof the maximum value of absorption and/ or an increase of the absorptioneffect.

The shapes of the two embodiments now described, that is a cone and acone with a cylindrical base, respectively, are in first line intendedto be made of non-ductile material such as cardboard or the like. If,instead, a material is used, such as aluminium foil or the like, whichis shaped into a curved configuration by means of a pressing procedureit will be possible to follow the same principle as previously describedin assembling the absorber, but in this case a peripheral edge 20 atevery mainly sector shaped unit 19 is already bent or pressed up duringthe manufacturing step, as best seen in Figure 5. Figure 6 shows anabsorber according to this design installed.

It will be easily appreciated that in assembling, for instance, threesuch units, a cone is obtained with a cylindrical or somewhat conicalbasis 20 without employing the previously described loose ring 17. Injoining together such an absorber, straight U-shaped clips 21, 22(Figure 5) may be used, being of a type somewhat differing from thefastening means previously described, wherein, for instance, a clip 22is fastened at each joint onto the folded abutting edges 24 of thecylindrical part 20 and three or more clips 21 are fastened onto theabutting edges 23 of the conical part 1. Instead of these clips also theclips 6 described may be employed.

It will be evident from the foregoing that it is desirable not only toobtain a varying ratio between the front panel surface and the airvolume behind the surface of the absorber at different spots thereof,but also that the air volume contained behind each unit area is great inrelation to the unit area itself. This condition is complied with, notonly by the cone and the combination of cone and cylinder but also byhollow bodies of revolution with a generatrix in the shape of a brokenline or a curved line, for instance, a hemisphere, a hemiellipsoid, orthe like, which in relation to the first mentioned bodies involve theadvantage that every part of their surface is curved in two planes whichrenders a greater rigidity than one-plane curvature. Hence, it will bepossible in employing this design to obtain the desired rigidity with athinner material than would be required by curving in one plane.

If the absorber is made as a spherical, ellipsoidal, or similar segmentsurface, it is suitably pressed integrally from a perforated basicmaterial, since units having this shape readily may be stacked onto eachother for transportation. If the absorber is designed as a hemisphere,hemiellipsoid, or the like, which cannot advantageously be stacked ontoeach other, it should preferably be made in two or three sections withcompletely inturned edges. These sections may be readily stacked ontoeach other for transportation, and when they are to be assembled thebent metallic clips 21, 22 are utilized, engaging the inturned edges.

In assembling as well as installing this embodiment, the same furthersteps should be taken as previously described in connection with thecone-shaped absorber. For aesthetical reasons, however, it would beadvisable not to design the small hood 8, positioned in the center ofthe absorber, as a truncated conical envelope surface but instead togive it the shape of a cup conforming to the shape of the absorber.

Any of the previously disclosed embodiments may have a layer of poroussound absorbing material applied therein, as shown in Figure 1 forexample.

The detailed description herein of the various embodiments of theinvention for the purpose of explaining the principles thereof is not tobe construed as restricting or limiting the invention, since manymodifications may be made by the exercise of skill in the art.

We claim:

1. A sound absorber for covering a surface, comprising a cup-shapedhollow revolution body of sheet material conically tapering in at leastone direction with respect to its axis of rotation and terminating in acircular edge in the opposite direction, at least said conicallytapering part being composed of annular sectors of perforated sheetmaterial curved relative to the plane of said sheet material and havingradially extending edge portions substantially perpendicular to saidcurved surface, said annular sectors being juxtaposed with said edgeportions in abutment in adjacent sectors so as to form a truncatedconical wall having two end openings of different diameter, resilientclenching means engaging remote sides of adjacent edge portions so as tomaintain the latter in abutment, resilient suspending means having oneend fixed relative to said surface and the other end provided withfastening means extending through one of said end openings having asmaller diameter, said fastening means engaging said cup-shaped body soas to resiliently maintain it in engagement with said surface along saidcircular edge.

2. A sound absorber for covering a surface, comprising a cup-shapedconical hollow body composed of annular sectors of perforated sheetmaterial curved relative to the plane of said sheet material and havingradially extending edge portions substantially perpendicular to saidcurved surfaces, said annular sectors being juxtaposed with said edgeportions in abutment in adjacent sectors so as to form a truncatedconical wall having two end openings of different diameter, resilientclenching means engaging remote sides of adjacent edge portions so as tomaintain the latter in abutment, resilient suspending means having oneend fixed relative to said surface and another end provided withfastening means extending through one of said end openings having asmaller diameter, said fastening means engaging a rim portion aroundsaid smaller diameter opening so as to resiliently maintain saidcupshaped body in engagement with said surface along a circular edgesurrounding said end opening having a greater diameter.

3. A sound absorber for covering a surface, comprising a cup-shapedhollow revolution body of sheet material conically tapering in onedirection of its axis of rotation, and having a cylindrical partadjacent to said conically tapering part terminating in a circular edgein the opposite direction, said conically tapering part being composedof annular sectors of perforated sheet material curved relative to theplane of said sheet material and having radially extending edge portionssubstantially perpendicular to said curved surface, said annular sectorsbeing juxtaposed with said edge portions in abutment in adjacent sectorsso as to form a truncated conical wall having two end openings ofdifferent diameter, said cylindrical part being connected with saidconically tapering part along an edge portion of the latter surroundingone of said end openings having a greater diameter, resilient clenchingmeans engaging remote sides of adjacent edge portions so as to maintainthe latter in abutment, resilient suspending means having one end fixedrelative to said surface and another end provided with fastening meansextending through the other of said end openings having a smallerdiameter, said fastening means engaging a rim portion around saidsmaller diameter opening so as to resiliently maintain said cup-shapedbody in engagement with said surface along the circular edge of saidcylindrical part.

4. A sound absorber for covering a surface, compris ing a cup-shapedhollow revolution body of sheet material conically tapering in onedirection of its axis of rotation and having a cylindrical part adjacentto said conically tapering part terminating in a circular edge in theopposite direction, said conically tapering part consisting of a hollowwall having the shape of an annular envelope confined between twoconcentric conical walls of different tapering angle and saidcylindrical part, said conical walls being composed of annular sectorsof perforated sheet material curved relative to the plane of said sheetmaterial and having radially extending edge portions substantiallyperpendicular to said curved surfaces, said annular sectors beingjuxtaposed with said edge portions in abutment in adjacent sectors so asto form two truncated conical walls of different tapering angle eachhaving two opposite end openings of different diameter said twotruncated conical walls being in engagement near said end openingshaving a smaller diameter while in engagernent with said cylindricalpart along their edge portions surrounding said end openings having agreater diameter, resilient clenching means engaging remote sides ofadjacent edge portions so as to maintain the latter in abutment,resilient suspending means having one end fixed relative to said surfaceand another end provided with fastening means extending through saidopenings having said smaller diameter, said fastening means engaging arim portion around said smaller diameter openings so as to resilientlymaintain said cup-shaped body in engagement with said surface along thecircular edge of said cylindrical part.

5. A sound absorber for covering a surface, comprising a cup-shapedhollow revolution body of sheet material conically tapering in onedirection of its axis of rotation and having a cylindrical partconnected to said conically tapering part terminating in a circular edgein the opposite direction, said hollow body being composed of sectionsof perforated sheet material each having a conical annular sector and acylindrical segment having a central angle which is the same as saidconical sector, said sections having edge portions substantiallyperpendicular to the surface of said conical sector and said cylindricalsegment, respectively, said sections being juxtaposed with said edgeportions in abutment in adjacent sections so as to form said cup-shapedhollow body and leaving an opening at the tapering end of said hollowbody, resilient clenching means engaging remote sides of adjacent edgeportions of adjacent sections so as to maintain the latter in abutment,and resilient suspending means having one end fixed relative to saidsurface and another end provided with fastening means extending throughsaid opening at the tapering end and engaging a rim portion around saidopening so as to resiliently maintain said cup-shaped body in engagementwith said surface along said circular edge of said cylindrical part.

6. A sound absorber according to claim 1, wherein porous material isintroduced into the absorber in order to increase its acousticimpedance.

References Cited in the file of this patent UNITED STATES PATENTS1,363,782 Handel Dec. 28, 1920 1,467,689 Scherer Sept. 11, 19231,509,744 Wilson Sept. 23, 1924 2,161,708 Heerwagen June 6, 19392,186,511 Welch Ian. 9, 1940 2,390,262 Mazer Dec. 4, 1945 2,610,695 GrueSept. 16, 1952 FOREIGN PATENTS 43,323 Denmark Oct. 27, 1930 111,603Sweden Aug. 29, 1944

